fix(session): restore busy route handling and add regression coverage (#20125)

packages/opencode/src/server/middleware.ts

@@ -1,6 +1,7 @@
 import { Provider } from "../provider/provider"
 import { NamedError } from "@opencode-ai/util/error"
 import { NotFoundError } from "../storage/db"
+import { Session } from "../session"
 import type { ContentfulStatusCode } from "hono/utils/http-status"
 import type { ErrorHandler } from "hono"
 import { HTTPException } from "hono/http-exception"
@@ -20,6 +21,9 @@ export function errorHandler(log: Log.Logger): ErrorHandler {
       else status = 500
       return c.json(err.toObject(), { status })
     }
+    if (err instanceof Session.BusyError) {
+      return c.json(new NamedError.Unknown({ message: err.message }).toObject(), { status: 400 })
+    }
     if (err instanceof HTTPException) return err.getResponse()
     const message = err instanceof Error && err.stack ? err.stack : err.toString()
     return c.json(new NamedError.Unknown({ message }).toObject(), {